Production of titanium



G. E. SNOW PRODUCTION OF TITANIUM Aug. 4, 1959 Filed July 1, 195a'INVENTOR. By George E. Snow ATTORNEYS United States Patent PRODUCTIONOF TITANIUM George E. Snow, Palmerton, Pa., assignor to The New JerseyZinc Company, New York, N .Y., a corporation of New Jersey ApplicationJuly 1, 1958, Serial No. 745,862

2 Claims. (Cl. 204-64) This invention relates to the production oftitanium and, more particularly, to the electrodeposition of metallictitanium on the surface of a cathode which is distal with respect to ananode, the anode and cathode being in direct bath-communication with oneanother within a fused salt bath.

In the aforementioned distal cathode deposition operation, a fusedchloride salt bath containing titanium trichloride and titaniumdichloride is electrolyzed between the anode and cathode while titaniumtetrachloride is substantially continuously introduced into andassimilated by the bath, the anode and cathode being in directcommunication with one another through the bath uninterrupted by aphysical barrier. The electrolytic conditions are so maintained betweenthe electrodes, and the bath composition is so maintained adjacent thatsurface of the cathode distal with respect to the anode, that metallictitanium is deposited predominantly on this distal surface of thecathode. titanium at the cathode, chlorine is liberated at the anode. Inorder to prevent this chlorine from escaping to the entire cellatmosphere, where it could re-enter the bath and react with the lowerchlorides and thus oxidize these chlorides back to the tetrachlorideform, a refractorysurfaced Wall is provided to define an anode domewithin which the chlorine is collected as it is liberated from theanode. The walls of the chlorine dome extend below the surface of thecell bath, and the dome is provided with an outlet line so that thecollected chlorine can be recovered outside of the cell.

The chlorine dome is advantageously constructed of corrosion-resistantmetal such as nickel lined with a still more corrosion-resistant surfaceof refractory brick. In the course of operating a cell with such achlorine dome, it has been repeatedly observed that sometimes the nickelstructure has been seriously corroded, particularly at the bath surfacelevel, and that at other times the refractory brick lining has beensufiiciently corroded to loosen the bricks. Although these varying typesof corrosion of the chlorine dome have occurred while the dome waselectrically isolated from both the anode and the cathode, I have foundthat the type of corrosion of the chlorine dome is related to theattained value of the open circuit back electromotive force measuredbetween the dome and the anode during the operation. That is, I haveobserved that when this back electromotive force falls below about 2volts the metallic structure of the chlorine dome is attacked exteriorlyof the chlorine atmosphere and that when the back electromotive forcerises above about 2.4 volts the refractory lining of the dome adjacentthe chlorine atmosphere is attacked. However, I have discovered that ifthe chlorine dome is made anodic or cathodic, as the operation mayrequire, so that the open circuit back electromotive force between thechlorine dome and the anode is maintained within the range of about 2 toabout 2.4 volts, there is no significant corrosion either of themetallic structure or of the refractory lining of the chlorine dome. Thechlorine dome back Concomitant with the deposition of electromotiveforce is maintained within this range, pursuant to my invention, byelectrically connecting the chlorine dome to the cathode when the backelectromotive force falls below 2 volts and by electrically connectingthe chlorine dome to the anode when the back electromotive force risesabove 2.4 volts. In each instance, the electrical connection is made tosupply a suflicient current to the dome for a sufiicient period of timeto restore the back electromotive force to and maintain it within theaforesaid range.

The molten salt baths which are useful in practicing the inventioncomprise one or more of the halides of the alkali metals and alkalineearth metals. Thus, the chlorides, bromides, iodides and fluorides ofsodium, potassium and lithium as well as the same halides of calcium,magnesium, barium and strontium may be used with advantage. However, inthe interest of simplifying the recovery of the halogen which isliberated at the anode during electrolysis, it is presently preferred touse only the chlorides of these metals. Although an individual halidemay be used as a single constituent bath, I prefer to use a combinationof these halides inasmuch as such combinations are characterized byrelatively lower melting points than the individual salts. It isparticularly advantageous, when using a combination of theaforementioned halides, to mix .these halides in proportionsapproximating a eutectic composition in order to obtain baths with lowmelting points. For example, I have used with particularly satisfactoryresults a eutectic mixture composed of 5 mol percent of sodium chloride,40 mol percent of potassium chloride and 55 mol percent of lithiumchloride, the resulting mixture having a melting point of about 350 C.Other useful eutectic mixtures are represented by the mixture composedof 48.5 mol percent of sodium chloride and 51.5 mol percent of calciumchloride having a melting point of 505 C. and by the mixture composed of24 mol percent of barium chloride, 35 mol percent of sodium chloride and41 mol percent of potassium chloride having a melting point of 552 C. Ofcourse, as in all other molten salt electrolytic methods for theproduction of metallic titanium, the. bath should be as completelyanhydrous as possible and should be compounded of salts of high purity.

The titanium tetrachloride is advantageously supplied to the bath byintroducing it directly into the molten bath either with or without acarrier gas such as argon, and the cell is advantageously tightly closedin order to control the cell atmosphere. The cell atmosphere should, ofcourse, be compartrnented to maintain separation between the atmosphereabove the portion of the bath into which the titanium tetrachloride isintroduced and the portion of the bath from which the chlorine isevolved at the anode, and this compartmentation is advantageouslyprovided by a chlorine dome such as that previously described. Thechlorine dome should, of course, be formed of a material, such asrefractory brick and the like, which is normally resistant to attack bythe chlorine and titanium tetrachloride. For practical purposes,however, the refractory is usually mechanically reinforced with a metalgrid or other structure enclosed within or backing the refractorystructure, and thismetallic structure may be used advantageously formaking an electrical contact to the chlorine dome pursuant to thepresent invention.

The cell electrodes should be constructed of material which will notintroduce extraneous elements into the fused bath. Thus, a nonmetallicanode such as graphite or carbon should be used, graphite having beenfound in practice to be wholly suitable for this purpose. Cathodes ofnickel, and preferably of corrosion-resistant nickel base alloys, areuseful in practicing the invention. At the prevailing cell temperature,the aforementioned cathode materials have been found not to contaminatethe deposited metallic titanium to any significant degree and may beused in solid or foraminous form.

The relative position between, and the arrangement of, the anode andcathode within the molten salt body should be such that (a) chlorineevolved'at the anode will rise in the body of molten bath Withoutentering the body of molten bath adjacent the distal surface of thecathode, (b) the body of molten bath between the anode and the proximalcathode surface and the body of molten bath adjacent the distal surfaceof the cathode are in communication With one another through amultiplicity of passages, and (c) the distance between the anode and theproximal cathode surface is sufficiently small to permitelectrolytically induced depletion of the titanium content of the moltenbath between these surfaces.

A number of arrangements of anode and cathode will assure theseconditions, and a variety of such arrangements is shown in the drawingsin the copending application of Reimert and Fatzinger, Serial No.441,324, filed July 6, 1954. However, a presently preferred cellarrangement for practicing the invention is shown in the accompanyingdrawing in which the single figure is a partial sectional elevation ofthe cell.

As shown in the drawing, a closed cell 1 is provided with a fused saltbath 2 in which a cylindrical cathode is nearly but not completelyimmersed. The deposition cathode comprises a cylindrical side wall bodyportion 3 closed at its lower end with an impervious bottom wall 4 butopen at its top end. The side wall portion 3 is composed advantageouslyof sheet material having a large number of small openings 5 punched inan area of the cylinder which is confined to an upper border below thelevel of the bath and above the bottom wall 4. The impervious bottomwall 4 and upper side wall portions 6, as well as the pervious centralportion of the cathode, are constructed of sheet metal composed of acorrosionresistant nickel-base alloy. This deposition cathode structureis secured to supporting rods 7 which project into the cell through thecell roof and which thus provide an electrical connection from anexternal source to the cathode structure positioned within the cell. Thesupporting rods are mounted in insulating material 8 as they passthrough the cell cover. The cathode assembly thus encloses an inner bodyportion A of the fused salt bath 2.

The anode assembly for the cell comprises a chlorine dome 9 extendingdownwardly into the interior of the side walls 3 of the cathodeassembly, the lower extremities of the dome being immersed in the fusedsalt bath 2. The chlorine dome 9 is advantageously constructed of acorrosion-resistant metal such as nickel in the form ofa main frame 10with an inner lining 11 of alumina brick held in place with refractorycement. The dome is secured to the cover of the cell by insulatingportions 12 which also support a graphite anode 13 extending into thecell and downwardly into the interior of the cylindrical cathodeassembly. The roof of the cell is provided with a port 14 to permitescape of chlorine gas from the surface of the bath within the dome 9,and the cell roof is also provided with a titanium tetrachloride inletline 15 so as to supply the tetrachloride, either with or without aninert carrier gas such as argon, to the lower portion of the main bodyportion B of the fused salt bath 2.

In the cell arrangement just described, the chlorine evolved at theanode 13 leaves the surface of the bath within the confines of thechlorine dome 9 which thus define a compartment C in the cell atmospherecontaining the evolved chlorine. The portion of the cell atmosphereexterior of this chlorine compartment defined by the cell wallscomprises a compartment D into or through which titanium tetrachlorideis introduced. It will beseen, accordingly, that the titaniumtetrachloride is absorbed by the body portion B of the bath and isassimilated only in that portion of the bath which is in contact withthe distal surface of the deposition cathode. The body portion A of thebath, on the other hand, is maintained substantially completely depletedof titanium ions by control of the electrolyzing conditions. Anyunabsorbed argon is withdrawn from compartment D through an exit line 16in the cell roof.

The electrolyzing condition which assures the maintenance oftitanium-depletion in the body portion A of the molten bath between theanode and the proximate cathode surface comprises the use of a voltagesufficiently high to strip the body portion A of the bath of itstitanium chloride content. When the body portion A is effectivelystripped of its titanium chloride content, thus leaving essentially onlythe aforementioned eutectic bath composition composed of lithium, sodiumand potassium chlorides, the back electromotive force of the cell, whenmeasured across the anode and cathode upon opening of the exterior cellcircuit, has a magnitude of about 2.6 volts or more when operating witha bath temperature of about 550 C. A back electromotive force belowabout 2.4 volts is an indication of the presence of titanium chloride inthe body portion A. As an upper limit of about 3.4 volts is exceeded,and particularly as the back electromotive force reaches about 3.5volts, decomposition of the non-titaniferous bath components such as thealkali metal chlorides begins to occur. The maximum back electromotiveforce will, of course, be influenced by the bath composition, by theelectrode compositions and by the bath temperature, but in general itcan be stated that under most conditions the presently preferred upperlimit for the back electromotive force is about 3.4 volts. The backelectromotive force is maintained within the aforementioned range eitherby control of the cell voltage, so as to maintain appropriate depletionof titanium ions in the body portion A of the bath, or, as described inthe copending application of Earl W. Andrews, Serial No. 628,117, filedDecember 13, 1956, by controlling the rate at which the titaniumtetrachloride is delivered to the cell for assimilation by the moltenbath. Measurement of the back electromotive force at intervals of 15minutes is generally sufficiently frequent to permit the maintenance ofa substantially uniform back electromotive force to within aboutone-tenth of a volt.

The back electromotive force between the chlorine dome 9 and the anode13 is maintained within the necessary range pursuant to my invention bymeans of the electrical connections schematically indicated in thedrawing. Thus, a line 17 connects the chlorine dome with a variableresistance 18 which in turn is connected to a switch 19. The poles ofthe switch are connected to the anode and cathode so that the circuitincluding the variable resistance 18 and the chlorine dome 9 may beconnected either to the anode or to the cathode. A voltmeter 2%) isconnected between the anode and the line 17 so as to measure the opencircuit back electromotive force between the chlorine dome and theanode. Thus, when the back electromotive force between the chlorine domeand the anode falls below about 2 volts, the switch 19 is operated toconnect the dome to the cathode and the variable resistance 18 isadjusted to supply sufiicient current to the dome to restore its backelectromotive force to the necessary range; when the back electromotiveforce between the dome and the anode rises above about 2.4 volts, theswitch 19 is operated to connect the dome to the anode, and again thevariable resistance 18 is adjusted to supply sufficient current to thedome to restore its proper back electromotive force. Whether thechlorine dome is connected to the anode or to the cathode, or iselectrically free of both, the value of the back electromotive forcebetween the dome and the anode is measured at appropriate intervals,generally at intervals of about 15 minutes, to determine whether theprevailing anodic or cathodic bias, or absence of any bias, should bechanged or whether the prevailing bias current value should be changed.

The effectiveness of the method of my invention for protecting thechlorine dome will be readily apparent from the following example inwhich operations with a dome back electromotive force above and belowthe proper range are compared with an operation embodying the invention.The operations were carried out in an eleven-inch diameter cell, similarto that shown in the drawing, partially filled with a eutectic saltmixture composed of 53.2% KCl, 41.6% LiCl, and 5.2% NaCl and heated byexternal means to 600 C. After sealing the cell and flushing it withargon, TiCl was fed below the melt in the catholyte compartment. At thesame time current was applied at a rate of 2 faradays per mol of TiCL;being fed. The extent to which the amperage (and the feed) could beincreased was governed by periodic measurements of the cell back Thiswas maintained at about 2.6 volts. The cylindrical nickel cathode (3)was 5 inches in diameter with inch diameter holes (5), and the graphiteanode (13) was 2 inches in diameter. The chlorine dome (9) consisted ofa nickel tube 4 inches in diameter and 8 inches long and was lined withvertical strips of alumina refractory /8 inch wide and 4 inch thick heldin place with refractory cement. In each run, metallic titanium of highquality was electrodeposited on the outer (distal) surface of thecathode.

The duration of one run was 45 hours and the average cell current was134 amperes. No attempt was made to control the dome back (i.e. the opencircuit voltage between the chlorine dome and the anode), but it wasmeasured at intervals with a voltmeter. During the first 12 hours of therun the dome back was 1.4 volts, but then it rapidly rose to 2.5 voltsand gradually increased to 2.9 volts during the remaining 33 hours ofthe run. At the end of the run the alumina lining was severely crackedat the melt level, and some of the cement had fallen out. Both thecement and the alumina were highly alkaline.

The duration of the next run was 42 hours and the average cell currentwas 139 amperes. Again, no attempt was made to control the dome back andit stayed within the range of 0.9-1.3 volts throughout the run. At theend of the run the alumina and refractory cement were both in perfectcondition, but there were deep grooves corroded into the nickel backingat the melt level. At one point below the melt level, there was adiameter hole completely corroded through the nickel.

The duration of a third run was 46 hours and the average current was 142amperes. The dome back was maintained at 2.2 volts throughout the run byusing a bias current pursuant to the invention. During most of the run,a cathodic bias current of 1 to 10 amperes was required, but during a/z-hour period an anodic current of 9 amperes was necessary. At the endof the run the dome lining was in perfect condition, and there was nocorrosion of the nickel backing.

Iclaim:

1. In the electrolysis between an anode and a cathode of a fused halidesalt bath containing a titanium tower chloride wherein titaniumtetrachloride is substantially continuously introduced into andassimilated by the bath and metallic titanium is deposited on thecathode, the anode and cathode being in direct communication with oneanother through the bath uninterrupted by any physical barrier, thetitanium metal being deposited predominantly on the surface of thecathode distal with respect to the anode, the chlorine evolved attheanode being collected in and discharged from a confined space definedby a refractory-surfaced metallic conductorcontaining wall extendingbelow the surface of the bath and normally electrically isolated fromboth the anode and cathode, and the titanium tetrachloride beingdelivered to the portion of the bath in contact with the distal surfaceof the cathode, the improvement which comprises maintaining theopen-circuit back electromotive force between the refractory-surfacedwall and the anode within the range of 2 to 2.4 volts by electricallyconnecting the wall to the cathode when the back electromotive forcefalls below said range and by electrically connecting the wall to theanode when the back electromotive force rises above said range, theelectrical connection being made in each instance to deliver asuflicient current to the wall for a suflicient period of time torestore the open-circuit back electromotive force to and maintain itwithin said range.

2. In the electrolysis between an anode and a cathode of a fused halidesalt bath containing a titanium lower chloride wherein titaniumtetrachloride is substantially continuously introduced into andassimilated by the bath and metallic titanium is deposited on thecathode, the anode and cathode being in direct communication with oneanother through the bath uninterrupted by any physical barrier, thetitanium metal being deposited predominantly on the surface of thecathode distal with respect to the anode, the chlorine evolved at theanode being collected in and discharged from a confined space defined bya refractory-surfaced metal Wall extending below the surface of the bathand normally electrically isolated from both the anode and cathode, andthe titanium tetrachloride being delivered to the portion of the bath incontact with the distal surface of the cathode, the improvement whichcomprises maintaining the opencircuit back electromotive force betweenthe refractorysurfaced wall and the anode within the range of 2 to 2.4volts by electrically connecting the wall to the cathode when the backelectromotive force falls below said range and by electricallyconnecting the wall to the anode when the back electromotive force risesabove said range, the electrical connection being made in each instanceto deliver a suflicient current to the wall for a sufiicient period oftime to restore the open-circuit back electromotive force to andmaintain it within said range.

References Cited in the file of this patent UNITED STATES PATENTS2,848,397 Reimert et a1. Aug. 19, 1958

1. IN THE ELECTROLYSIS BETWEEN AN ANODE AND A CATHODE OF A FUSED HALIDESALT BATH CONTAINING A TITANIUM LOWER CHLORIDE WHEREIN TITANIUMTETRACHLORIDE IS SUBSTANTIALLY CONTINUOUSLY INTRODUCED INTO ANASSIMILATED BY THE BATH AND METALLIC TITANIUM IS DEPOSITED ON THECATHODE, THE ANODE AND CATHODE BEING IN DIRECT COMMUNICATION WITH ONEANOTHER THROUGHT THE BATH UNINTERRUPTED BY ANY PHYSICAL BARRIER, THETITANIUM METAL BEING DEPOSITED PREDOMINANTLY ON THE SURFACE OF THECATHODE DISTAL WITH RESPECT TO THE ANODE, THE CHLORINE EVOLVED AT THEANODE BEING COLLECTED IN AND DISCHARGED FROM A CONFINED SPACE DEFINED BYA REFRACTORY-SURFACED METALLIC CONDUCTORCONTAINING WALL EXTENDING BELOWTHE SURFACE OF THE BATH AND NORMALLY ELECTRICALLY ISOLATED FROM BOTH THEANODE AND CATHODE, AND THE TITANIUM TETRACHLORIDE BEING DELIVERED TO THEPORTION OF THE BATH IN CONTACT WITH THE DISTAL SURFACE OF THE CATHODE,THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE OPEN-CIRCUIT BACKELECTROMOTIVE FORCE BETWEEN THE REFRACTORY-SURFACE WALL AND THE ANODEWITHIN THE RANGE OF 2 TO 2.4 VOLTS BY ELECTRICALLY CONNECTING THE WALLTO THE CATHODE WHEN THE BACK ELECTROMOTIVE FORCE FALLS BELOW SAID RANGEAND BY ELECTRICALLY CONNECTING THE WALL TO THE ANODE WHEN THE BACKELECTRO-